Besides reactive minerals, aggregates may contain other impurities, such as organic matter, which are harmful to concrete.
Organic matter, such as that derived from decaying vegetation, is capable of delaying setting and hardening of concrete. It is more likely to be found in fine than in coarse aggregate and may be detected by the test set out in NZS 3111:1986 Methods of Test for Water and Aggregate for Concrete, Section 17. In this test, sand in a bottle is inundated in a sodium hydroxide solution and allowed to stand for 24 hours. The colour of the liquid above the sample is then compared with the colour of a standard reference solution. If the colour of the liquid is lighter than that of the reference solution, the amount of organic impurities present in the aggregate is not significant. If the colour of the liquid is darker than that of the reference solution, the aggregate contains organic compounds and further tests should be made to determine if these are harmful. Normally, the strength of concrete made with the sand is used as a gauge of the harmful effects of the impurities.
Sugar has a strong retarding effect on the setting and hardening of concrete. In severe cases of contamination, the resulting concrete may not set or may fail to gain appreciable strength. AS 2758.1:2014 Aggregates and Rock for Engineering Purposes - Part 1: Concrete Aggregates specifies a maximum limit on the sugar content of aggregate of one part in 10,000 determined in accordance with AS 1141.5-2000 (R2016) Methods for Sampling and Testing Aggregates Particle Density and Water Absorption of Fine Aggregate.
Silt, clay and dust may form a coating on aggregate particles, resulting in weakened bond between the aggregate and the cement paste. Excessive amounts of these fine materials may also increase unduly the water demand of the concrete, resulting in loss of concrete strength and an increase in its permeability. Cleanness in coarse aggregate can be determined by a test contained in NZS 3111, Section 13. Similarly the same Standard in Section 18 has a method of determining clay content.
The presence of certain clay minerals, particularly the montmorilionites, will cause changes in volume with changing moisture conditions. Where such volumetric instability exists, the concrete may deteriorate rapidly with cycles of wetting and drying. The wet/dry strength-variation test set out in AS 1141.22-2008 Methods for Sampling and Testing Aggregates: Wet/Dry Strength Variation can be used as a guide to the dimensional stability of the coarse aggregate.
The amount of fine material is determined by washing a sample of the aggregate over a small sieve. Sections 12 and 13 in AS 1141 describe this type of test.
Coal, wood and other lightweight materials tend to rise to the surface during vibration of concrete, especially in pavements and floors, and produce a very poor surface finish. They also cause pop-outs and staining on vertical surfaces. The percentage of light particles can be determined by the test set out in NZS 3111 Section 9. AS 2758.1:2014 Aggregates and Rock for Engineering Purposes: Concrete Aggregates specifies a maximum limit on light particles of 1 % by mass of aggregate (3% for slag aggregate).
There are no limits set by New Zealand Standards but as the percentage of lightweight particles increases the strength will decrease, requiring the use of NZS 3111, Section 8 to determine an acceptable limit for supply of concrete. Where surface appearance of the concrete is important, the amount of coal, wood and charcoal should preferably be even less.
Aggregates, particularly those dredged from the sea, or those quenched and washed with sea water, may be contaminated by sea salt which contains a high proportion of chloride ions. The amount of chlorides in concrete is of major concern because of its influence on the corrosion of embedded steel. They also increase shrinkage and reduce the sulphate resistance of concrete. Table 8 in AS 2758.1 and Clause 6.6 of NZS 3109:1997 Concrete Construction specify (in different ways) maximum chloride contents for concrete as placed. Adoption of the limits specified in the latter is recommended, i.e. total chloride of concrete shall not exceed for:
| Prestressed Concrete
| Reinforced Concrete
| Located in moist environment or exposed to chloride
| Located in a dry or protected environment
| Total sulphate content shall not exceed 5% of the mass of the cement.